Risk analysis theory mainly refers to use scientific methods to reduce losses resulting from all kinds of uncertainties to the minimum through differentiation, estimation and disposal. Based on the theory, this article discusses the risk problems in grain yield. Given a grain yield sequence, firstly we decompose it into two main parts according to different factors that affect yield. Secondly we translate the two parts into a ratio sequence, through which we could get rid of the influence of dimension as well as productivity, and the sequence reflects the information of fluctuation of grain yield during recent years. In addition, we form a function to simulate the rate sequence and subsequently calculate the probability of a cerain risk level of the sequence using the method of integral. This kind of information conduces to disaster management as well as formulation of proper policies in disaster reduction.